In an image forming device, such as a laser printer, a projector, or an image scanner, a scanning optical device, such as a two-dimensional scanning device which scans a light beam irradiated from a light source on a surface to be scanned or a multi-beam scanning device, is mounted. In such type of scanning optical device, an optical configuration is adapted in which a relay optical system for adjusting condensing positions is arranged, so as to once condensing the light beam on a deflector arranged in the middle of an optical path extending from the light source to the surface to be scanned. The relay optical system includes a plurality of relay lenses. It is a cause of increasing a size of the scanning optical device. In order to reduce the size of the scanning optical device, folding mirrors for folding the optical path are arranged between the relay lenses of a generic relay optical system.
A focal length of a relay optical system tends to be set to short in comparison with that of a subsequent optical system. Therefore, in order to form an image which is sufficiently focused on an irradiated surface (a surface to be scanned), it is necessary that points of focus of relay lenses are mutually aligned at a high precision. Further, in order to cause the light beam to efficiently enter the relay optical system, or, in order to cause a light beam emitted from the relay optical system to efficiently enter the subsequent optical system, it is necessary to determine a positional relationship among (an entrance pupil or an exit pupil of) the relay optical system and its front and rear optical systems with a high precision. However, it is practically difficult to position the relay lenses with respect to each other with a high precision, while simultaneously determining the positional relationship among the front and rear optical systems with a high precision. For example, when distances between the relay lenses are adjusted through moving the relay lenses so as to adjust the points of focus of the relay lenses with respect to each other, an adverse effect arises such that a conjugate relationship of pupils in front and at rear of the relay optical system changes.
A method of adjusting the points of focus of the relay lenses with respect to each other can be considered such that distances between relay lenses are adjusted by performing a shift adjustment of the folding mirrors, while the relay lenses are fixed. When such a method is adapted, there is an advantage that the positional relationship among the relay optical system and its front and rear optical systems are not changed, since it is not necessary to move the relay lenses themselves, during the adjustment of the points of the relay lenses. On the other hand, in accordance with the shift adjustment of the folding mirrors, the incident positions (reflection positions) of the light beam on the folding mirrors are fluctuated. Another adverse effect arises such that an incident height of the light beam to the subsequent optical systems is fluctuated, in association with the movement the reflection positions of the light beam on the folding mirrors with respect to the relay lenses. Further, when the folding mirrors are inclined by fluctuations of components during the shift adjustment of the folding mirrors, an adverse effect arises such that an angle of the light beam emitted from the relay lenses to the subsequent optical systems is varied.
To prevent such adverse effects from arising, it is considered to adjust the points of focus of the relay lenses by adapting a focus adjustment mechanism disclosed in, for example, Japanese Patent Provisional Publication No. HEI11-196236 (hereinafter, written as patent document 1). The focus adjustment mechanism disclosed in the patent document 1 is configured such that points of focus of the scanning optical system with respect to a document surface is adjusted through adjusting distances among first mirrors installed in respective two mirror supports guided by guide rails and a pair of second mirrors.
However, in the focus adjustment mechanism described in the patent document 1, for example, when the distance between the mirrors is changed by ΔD, the optical length is changed with a twice sensitivity, that is, changed by Δ2D. Therefore, such a focus adjustment mechanism is not suitable for finely adjusting the points of focus of the relay lenses.